Hybrid and electric vehicles (HEVs) typically include an electric traction drive system that includes at least one alternating current (AC) electric motor which is driven by a power inverter module (PIM) with power from a direct current (DC) power source, such as a storage battery. Motor windings of the AC electric motor can be coupled to inverter sub-modules of the PIM. Each inverter sub-module includes a pair of switches that switch in a complementary manner to perform a rapid switching function to convert the DC power to AC power. A pulse width modulation (PWM) module receives voltage command signals and applies PWM waveforms to the voltage command signals to control pulse width modulation of the voltage command signals and generate switching vector signals that are provided to the inverter sub-modules of the inverter module. When the switching vector signals are applied, each pair of switches in each of the inverter sub-module switch in a complementary manner to perform a rapid switching function to convert the DC power to AC power. This AC power drives the AC electric motor, which in turn drives a shaft of HEV's drivetrain.
Vector Control
Many modern high performance AC motor drives use the principle of field oriented control (FOC) or “vector” control to control operation of the AC electric motor. In particular, vector control is often used in variable speed drives to control currents fed to an AC electric motor so that the torque applied to a shaft by the AC electric motor can be controlled and hence the mechanical angular velocity of motor's rotor can be controlled. In short, stator phase currents are measured and converted into a corresponding complex space vector. This current vector is then transformed to a coordinate system rotating with the rotor of the AC electric motor.
To ensure proper control of the electric machine, vector control requires angular position information of the rotor (i.e., the mechanical rotational angular position of rotor relative to the “stator” or motor windings).
Traditional motor control systems normally include a feedback device (e.g., angular position or speed sensor) to provide angular position or frequency information about the motor. For example, many vector controlled motor drive systems employ a rotor speed or position sensor to provide information regarding the rotor's angular position that is needed to control the motor. For instance, the rotor's angular position can be computed based on actual measured quantities using some type of speed or position sensor for control feedback measurement. As one example, to determine the angular position of the rotor, its angular speed can be measured with a speed sensor, and the angular position can then be obtained by integrating the speed measurements. Other field-oriented or vector controlled systems may use a rotor angular position sensor or rotational transducer that provides absolute position information directly to implement motor control techniques. One such example would be a resolver and resolver-to-digital converter circuit, which directly provides position information that corresponds to the rotor's angular position.
It is important that the sensors have sufficient accuracy and resolution, and therefore, many systems employ high resolution sensors to help improve system performance. At the same time, the cost of feedback devices and associated interface circuits is significant, and therefore it is desirable to reduce cost of these devices. As such, in some cases, it may be desirable to employ low resolution sensors since they are less expensive.
However, when the resolution of the sensor is low, the computed and/or the estimated angular position can have substantial ripples and errors. This can degrade control performance (e.g., in terms of torque accuracy/linearity that can be achieved).
It would be desirable to provide improved methods, systems and apparatus for estimating angular position and angular velocity of an electric machine's rotor that can allow for the quality of the estimated angular position and angular velocity to be improved. It would also be desirable if such improved methods and systems for estimating angular position and angular velocity can improve torque accuracy such that the actual output torque generated closely tracks commanded torque. It would also be desirable if such methods, systems and apparatus can allow for use of low resolution sensors and thus decrease cost of systems that rely on sensors to estimate angular velocity and/or angular position. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.